Code 121 = Heat exchanger overheat. Safety thermistor detected temperature above safe limit. Serious code.
Almost always: scaled heat exchanger
Most common cause. Scale insulates exchanger; burner can't transfer heat fast enough; metal temperature rises. Fix: full descale.
Other causes
- Restricted combustion air or exhaust
- Low water flow during draw (clogged inlet filter)
- For T-T cascade: heating loop circulator issue
Reset
- Power off completely; wait 30 minutes to cool
- Power back on, attempt reset
- Don't cycle if code returns — underlying cause active
If 121 persists
Call a Takagi-trained installer. Repeated overheat events cause cumulative damage.
Prevention
- Annual descaling
- Water softener above 11 gpg
- Annual inlet filter cleaning
Bottom line
Code 121 is the cost of skipping annual maintenance. Descale, then call an installer if it returns.